SCR is the acronym of Silicon Controlled Rectifier, as the name suggests it's a kind of diode or a rectifying agent whose conduction or operation can be controlled through an external trigger.
It means that this device will switch ON or OFF in response to an external small signal or voltage, quite similar to a transistor but yet hugely different with technical characteristics.
Looking at the figure we can see that a SCR has three leads which mat be identified as follows:
Keeping the printed side of the device facing us,
The right end lead is called the "gate".
The center lead is the "Anode", and
The left end lead is the "Cathode"
How these should be connected:
The gate is the trigger input of an SCR and requires a DC trigger with a voltage of around 2 volts, the DC should be ideally more than 10mA. This trigger is applied across the gate and the ground of the circuit, meaning the positive of the DC goes to the gate and the negative to the ground.
The conduction of voltage across the anode and the cathode is switched ON when the gate trigger is applied and vice versa.
The extreme left lead or the cathode of an SCR should always be connected to the ground of the triggering circuit, meaning the ground of the triggering circuit should be made common by connecting to the SCR cathode or else the SCR will never respond to the applied triggers.
The load is always connected across the anode and an AC supply voltage which may be required for activating the load.
SCRs are specifically suited for switching AC loads, DC loads will not work with SCRs because with a DC load since the SCR is triggered, it will latch and will not switch OFF even after the gate trigger is removed.
The following examples shows a few application circuits which can be built and also modified for more useful purposes:
Simple Rain Alarm:
The above circuit of a rain alarm can be used for activating a AC load, like a lamp or an automatic folding cover or shade.
The sensor is made by placing to metallic pegs, or screws or similar metal over a plastic body. The wires from these metals are connected across the base of a triggering transistor stage.
The sensor is the only part of the circuit which is placed outdoors, for sensing a rain fall.
When a rain fall begins, water droplets bridge the metals of the sensor.
Small voltage start leaking across the sensor metals and reach the base of the transistor, the transistor immediately conducts and supplies the required gate current to the SCR.
The SCR also responds and switches ON the connected AC load for pulling an automatic cover or simply an alarm for correcting the situation as desired by the user.
Simple Burglar Alarm
We discussed in the previous section regarding a special property of SCR where it latches in response to DC loads.
The circuit described below exploits the above property of the SCR effectively for triggering an alarm in response to a possible theft.
Here, initially the SCR is held in a switched OFF position as long as its gate stays rigged or screwed with the ground potential which happens to be the body of the asset which is required to be protected.
If an attempt to steal the asset is made by unscrewing the relevant bolt, the ground potential to the SCR is removed and the transistor gets activated through the associated resistor connected across its base and positive.
The SCR also instantly triggers because now it gets its gate voltage from the transistor emitter, and latches sounding the connected DC alarm.
The alarm remains switched ON until its switched OFF manually, hopefully by the actual owner.
Simple Fence Charger, Energizer Circuit
SCRs becomes ideally suited for making fence charger circuits. Fence chargers primarily require a high voltage generator stage, where a high switching device like an SCR becomes highly imperative. SCRs thus become specifically suitable for such applications where they are used for generating the required high arcing voltages.
CDI Circuit for Automobiles:
As explained in the above application, SCRs are also widely used in automobiles, in their ignition systems. Capacitive discharge ignition circuits or CDI systems employ SCRs for generating high voltage switching required for the ignition process or for starting a vehicle ignition.